Piezoelectric resonator and method for manufacturing the same

ABSTRACT

In the case where an ultraminiature piezoelectric substrate, which has a resonating portion formed by making a concavity by etching in the surface of the piezoelectric substrate made of an anisotropic crystal material, is mass-produced by batch operation using a large-area piezoelectric substrate wafer, an annular portion surrounding each concavity is formed sufficiently thick to prevent cracking from occurring when the wafer is severed. A piezoelectric substrate  2  of an anisotropic piezoelectric crystal material has a thin resonating portion  4  and a thick annular portion  5  integrally surrounding the outer marginal edge of the resonating portion to form a concavity  3  in at least one of major surfaces of the substrate; the inner wall  5   a  of the annular portion in the one crystal orientation slopes gently more than the inner wall in the other crystal orientation perpendicular to said one crystal orientation, and the piezoelectric substrate is longer in said one crystal orientation than in the other crystal orientation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to improvements in a piezoelectricsubstrate of a structure having an ultra-thin resonating portionsurrounded with a thick annular marginal portion formed integrallytherewith, a piezoelectric resonating element having a conductivepattern including excitation electrodes and so on formed on thepiezoelectric substrate, a piezoelectric resonator having thepiezoelectric resonating element hermetically sealed in a package, apiezoelectric oscillator using the piezoelectric resonator, and apiezoelectric substrate wafer; more particularly, the invention concernsa technique which in the case of forming the resonating portion profiledwith a concavity formed by etching in the surface of a piezoelectricsubstrate of an anisotropic crystal material, implements the optimumconfiguration of the resonating portion in conformity toultraminiaturization of the piezoelectric substrate through utilizationof an unetched portion (gentle slopes) on an inner wall of the annularmarginal portion, and a technique which increases mass productivity bybatch production while maintaining quality. Furthermore, the inventionpertains to a method suitable for fine adjustments to the thicknesses ofthe resonating portions formed by the bottom portions of a plurality ofconcavities prepared by one operation in a piezoelectric substratewafer, and a technique for providing the widest possible area for theresonating portion in a limited narrow piezoelectric substrate.

[0003] 2. Background Art

[0004] [First Prior Art Example]

[0005] Surface-mount piezoelectric devices of the type having a crystalresonator or similar piezoelectric resonating element hermeticallysealed in a package are used as a reference frequency source, a filterand so forth in communication equipment such as a portable telephone anda pager, and in electronic equipment such as a computer; and as thesepieces of equipment become miniaturized, there is also growing a demandfor miniaturization of the piezoelectric devices.

[0006] Furthermore, a piezoelectric oscillator for use as asurface-mount piezoelectric device has a structure in which apiezoelectric resonating element and parts forming an oscillationcircuit are housed in a concavity formed in the top surface of thepackage body as of ceramics material and sealed therein by covering theopen top of the concavity with a metal cover.

[0007] As the piezoelectric resonating element for use in such apiezoelectric device as mentioned above, there has been known apiezoelectric resonating element composed of: a piezoelectric substratewhich has, for high-frequency operations, a thin resonating portionformed by the bottom of a concavity formed by removing part of thesubstrate surface and surrounded with a thick annular marginal portionintegral therewith; input/output electrodes and a grounding electrodeformed on top and bottom surfaces of the resonating portion (Pat.Laid-Open Gazette No. 9-055635).

[0008] FIGS. 15(a) and (b) are a perspective and a sectional viewshowing the configuration of an AT-cut crystal resonating element as anexample of such a piezoelectric resonating element. The crystalresonating element, denoted generally by 100, is provided with: acrystal substrate 101 formed of an AT-cut crystal as an anisotropicpiezoelectric crystal material; excitation electrodes 110 formed on bothmajor surfaces of the crystal substrate; lead electrodes 111 extendingfrom the excitation electrodes 110; and connecting pads 112 formingrespective lead electrode terminating ends. The crystal substrate 101has a construction in which an ultra-thin resonating portion 103 isformed by the bottom of a concavity 102 made by etching in one of twomajor surfaces of a rectangular, flat-shaped substrate body longer inthe x-axis direction and the outer marginal edge of the resonatingportion 103 is integrally held by a thick annular portion 104. That oneside 104A of the annular marginal portion 104 lying in the x-axisdirection is extended a predetermined length in the x-axis direction toform a jut-out portion 105. On one surface of the jut-out portion 105the lead electrodes 111 are route thereto and the connecting pads 112are disposed at the ends of the lead electrodes 111.

[0009] The reason for making the AT-cut crystal substrate 101 longer inthe x-axis direction as mentioned above is that the propagation velocityof waves in the X-axis direction during excitation is approximately 1.2times higher than the propagation velocity of waves in the x-axisdirection; it is customary in the art to adopt such an x-axis longstructure which is longer in the x-axis direction.

[0010] In the case of using chemical etching to form the concavity 102in the crystal substrate 101, gentle slopes 104 a and 104 b of smallinclination angles θ are left unetched on inner walls of the annularmarginal portions 104 lying in the z-axis direction due to the propertyof the crystal as an anisotropic crystal material.

[0011]FIG. 15(c) is a sectional view showing the state in which thecrystal resonating element 100 of the above-described structure ismounted in a surface-mount package 120, wherein the connecting pads 112of the crystal resonating element 100 with the concavity 102 facingdownward are electrically and mechanically fixed by conductive adhesive122 to pads 121 disposed on the inner bottom of the package 120. The topopening of the package 120 is hermetically sealed with a metal cover123.

[0012] Incidentally, in the case of mass-producing such crystalsubstrates 101 (or crystal resonating element 100) by batch productionthrough use of a large-area piezoelectric substrate wafer, arrays ofindividual crystal oscillating elements 100 are laid out as shown inFIG. 16. That is, plural straight dicing grooves (dividing grooves) 131are cut in a wafer 130 in a grid pattern so that they cross one anotherat right angles, and rectangular areas defined by the grooves ultimatelybecome individual crystal substrates 101. Through etching of the wafer130 by use of a predetermined etchant with a mask (a resist film)through which is exposed the crystal substrate surface where theconcavity 102 will ultimately be formed, the gentle slopes 104 a and 104b are left unetched on the inner walls lying in the z-axis directioncorresponding to the crystal orientation in which the etching rate islow, as shown. Thereafter, the excitation electrodes 110, the leadelectrodes 111 and the connecting pads 112 are formed in the individualsubstrate regions as by vapor deposition, after which the wafer issevered along the dicing grooves 131 into individual crystal resonatingelements 100.

[0013] Incidentally, the crystal substrate to be housed in anultraminiature package measuring 2.5×2.0 mm needs to be further shrunkto a size of less than 1.3×0.9 mm. On the other hand, in batchproduction using the wafer 130 it is necessary that the individualcrystal substrates be closely spaced to enhance mass productivity byincreasing the number of crystal substrates obtainable from each wafer,but in the fabrication of such ultraminiature crystal substrates asmentioned above the spacing w between the dicing groove 131 and each ofthree marginal edges of the concavity 102 is extremely narrow, making itdifficult to provide a sufficiently broad and sufficiently strongannular marginal portion 104. Accordingly, in the case of cutting thewafer along the dicing grooves 131 by means of a dicing blade or similarcutting means, cracking readily occurs in the annular marginal portion104 and the resonating portion 103, giving rise to a problem of sharpreduction in productivity.

[0014] Moreover, since the lead electrodes 111 extending from theexcitation electrodes 110 formed on the top and bottom surfaces of theresonating portion 103, respectively, need to be routed along the innerwall of the steeply sloped one side 104A of the annular marginal portion104 located in the x-axis direction as depicted in FIG. 15(a), theconductive traces are readily broken at sharp marginal edges.

[0015] Besides, as shown in FIG. 15(c), the connecting pads 112 areformed on the jut-out portion 105 contiguous to the side 104A with thesteeply sloped inner wall and bonded to the pads 121 on the inner bottomof the package by use of the conductive adhesive 122, and consequently,the entire crystal resonating element structure is supported in acantilever fashion; in this case, however, since the distance betweenthe position where the connecting pads are bonded by the conductiveadhesive 122 and the resonating portion 103 is short, stress due to theweight of the crystal resonating element is likely to be applied to theresonating portion 103 to distort it, causing resonance frequencyvariations.

[0016] [Second Prior Art Example]

[0017] FIGS. 17(a) and (b) are a sectional view of another conventionalsurface-mount crystal resonator and a sectional view taken on the lineA-A, the resonator having a configuration in which the package 120having housed therein the crystal resonating element 100 held in acantilever fashion is hermetically sealed with the metal cover 123. Onboth sides of the jut-out portion 105 of the crystal substrate 101 thereare formed two connecting pads 112 a and 112 b, respectively. In thisinstance, the connecting pad 112 a facing toward the inner bottom of thepackage can easily be connected electrically and mechanically to the pad121 a opposite thereto by the conductive adhesive 122, but theconnection of the other connecting pad 112 b to the corresponding pad121 b in the package requires double coating of the adhesive since theformer is formed on the flat surface of the crystal substrate. Thedouble coating of the adhesive involves first coating of the adhesivebetween the pad 121 b and the underside of the crystal substrate andsecond coating of the adhesive to interconnect the upper connecting pad112 b and the adhesive coated first.

[0018] When coated twice, however, the adhesive 122 partly protrudesupward of the connecting pad 112 b, and to prevent it from contactingthe underside of the metal cover 123, it is necessary to increase theheight of the outer peripheral wall of the package 120. This constitutesan obstacle to a reduction in profile of the package and ignores thedemand for miniaturization.

[0019] As a solution to this problem, it is conventional to employ sucha structure as shown in FIG. 17(c), in which a concave notch 140 (140 a,140 b) deposited over the entire area of its inner wall with aconductive film is formed in the edge face of the substrate adjacent themarginal edge of the upper connecting pad 112 b to establish electricalconnections between the conductive film on the inner wall of the concavenotch 140 b and the connecting pad 112 b on the top of the substrate,whereas on the underside of the substrate there is formed a connectingpad 112 b′ for electrical connection with the conductive film on theinner wall of the concave notch 140 b. With this structure, the lowerconnecting pad 112 b′ and the pad 121 b on the inner bottom of thepackage are connected via the conductive adhesive 122, establishingelectrical connections between the upper connecting pad 112 b and thepad 121 b by single coating of the adhesive.

[0020] Such a concave notch as mentioned above is formed using theprocedure as shown in FIG. 17(d): making small rectangular holes in eachcrystal substrate 101 from both of its top and underside surfaces bychemical etching using a mask (resist film) for the large-areapiezoelectric substrate wafer 130; interconnecting the both smallrectangular holes to form a through hole 140H; depositing the conductivefilm all over the inner wall of the through hole; and severing the waferalong the dicing grooves 131 into individual crystal substrates.However, the diameter (width) of each through hole 140H to be formedwithin the width of the connecting pad 112 b on the ultraminiaturecrystal resonating element measuring, for instance, less than 1.3×0.9 mminevitably becomes as small as on the order of μ—this causes frequentoccurrence of insufficient etching that does not completely interconnectthe small rectangular holes made in each crystal substrate from its topand underside surfaces. On the other hand, since the through hole 140Hforming the concave notch 140 is formed in a narrow area of theconnecting pad 112 b of a limited area, the diameter of the hole is alsolimited accordingly. It is particularly difficult to form two throughholes in one marginal edge face of each piezoelectric substrate of anextremely small area.

[0021] Accordingly, there has been a strong demand for solving theproblem of low yields of ultraminiature crystal resonating elementscaused by insufficient chemical etching of the large-area piezoelectricsubstrate wafer 130 to form therein the through holes 140H which areultimately used as the concave notches 140.

[0022] Incidentally, the reason for providing the pair of concavenotches 140 in the edge face of each crystal substrate 101 is that theone concave notch 140 b is to establish electrical connections betweenthe upper connecting pad 112 b and the pad 121 b on the package asreferred o above, whereas the other concave notch 140 a is to provide onthe top surface of the crystal substrate the upper connecting pad 112 a′electrically connected to the lower connecting pad 112 a. With such anarrangement, measurement of characteristics of each individual crystalresonating element formed on the wafer 130 can be done with probe pinsof a measuring instrument held from the same direction against the twoconnecting pads 112 b and 112 a′ on the top of the crystal substrate orthe two connecting pads 112 a and 112 b′ on the underside of thesubstrate. The reason for this is that it is most efficient to conductthe measurement with the probe pins held against the two connecting padson the same surface of the substrate.

[0023] Besides, the crystal resonating element 100 is not always mountedin the package with the concavity oriented downward as shown in FIG.17(a), but it may also be held upward. Hence, the provision of the twoconnecting pads on either side of the substrate enables one crystalresonating element 100 to be mounted in the package in an arbitraryorientation.

[0024] [Third Prior Art Example]

[0025] In the formation of concavities by chemical etching in individualpiezoelectric substrate regions on a sheet-like piezoelectric substratewafer with a plurality of piezoelectric substrates arranged in a matrixform, it is difficult to make uniform the thicknesses of all ultrathinresonating portions formed by concavity bottom portions. To obviate thisproblem, it is customary in the prior art to premeasure the depth ofeach concavity, that is, variations in the thicknesses of the resonatingportions in the respective concavities, and to conduct an adjustmentoperation using an etchant for each concavity to make fine adjustment tothe thickness of the resonating portion having not reached apredetermined value.

[0026] FIGS. 18(a) and 18(b) are diagrams for explaining a conventionalfine adjustment method for each concavity, according to which theconcavities 102 are formed by simultaneously etching only those wafersurface areas exposed through apertures of a mask (resist film) coveringthe one major surface of the piezoelectric substrate wafer 130, thoughnot shown. Since such one operation by etching does not make uniform thethicknesses of the resonating portions 103 formed by the bottom portionsof the concavities 102, the thicknesses of the resonating portions 103of the concavities 102 are premeasured, and then etching is carried outfor each concavity after a guide mask with apertures arranged in a gridpattern, such as denoted by reference numeral 150, is mounted on thewafer 130 and held in close contact with the wafer surface areas betweenadjacent concavities. That is, the guide mask 150 has a plurality ofapertures 152 of a rectangular or some other shape formed through asheet of resin, for instance, with a predetermined pitch; the apertures152 of the shape matching the plan configuration of the concavities 150are defined by adjacent partitioning parts 151 intersecting in a gridpattern. The guide mask 150 is fixed to the wafer 130 all over it withthe partitioning parts 151 held in close contact with the wafer surfacesaround the concavities 102 as shown in FIG. 18(b). Then, the concavitiesare sequentially filled with proper amounts of etchant 155 for differentperiods of time precalculated therefor in decreasing order of thicknessof the resonating portion. At a point in time all the resonatingportions have been etched to a predetermined thickness, the entire waferassembly is cleaned up to remove therefrom the etchant.

[0027] Incidentally, miniaturization of the apertures 152 of the guidemask 150 is limited due to limitations imposed on machining techniques;and an achievable minimum size is such as depicted in FIG. 18(b).Accordingly, in order to make fine adjustments to the thicknesses of theresonating portions of miniature concavities 102 in the wafer 130 havingmore miniature piezoelectric substrates by individual etching as shownin FIG. 18(c), there is no choice but to use the guide mask 150 preparedfor large concavities. Alternatively, to permit accurate dropwisefilling of the concavities with the etchant, the apertures 152 need tobe of such a size as shown in FIG. 18(b) at minimum. When such a guidemask is used, the concavities 102 and the dicing grooves 131 are exposedthrough the apertures 152 defined by adjacent partitioning parts 151, asdepicted in FIG. 18(b). In this instance, when each aperture 152 isfilled with the etchant, excess etchant overflowing the concavitypenetrates into the dicing grooves 131 and unnecessarily etches awaythose regions undesired to etch, incurring a decrease in the mechanicalstrength of the regions concerned. Moreover, there is a fear that thesurface tension of the etchant 155 filling the concavity 102 prevents itfrom making full contact with the entire area of the bottom of theconcavity, leaving therein unetched portions 156 as shown in FIG. 18(d),and hence resulting in the individual etching becoming unsuccessful.

[0028] As described above, in the case of individual etching of theconcavities to remove variations in the depths of the concavities formedby batch operation in the wafer, the limitations on the size of theapertures of the guide mask 150 incur the possibility of unnecessaryetching or poor etching of the resonating portion.

[0029] [Fourth Prior Art Example]

[0030]FIG. 19 is a sectional view showing the configuration of an AT-cutcrystal substrate as an example of the piezoelectric substrate. Thecrystal substrate 101 is made of an AT-cut crystal as an anisotropicpiezoelectric crystal material, and the crystal substrate 101 has formedin both major surfaces thereof concavities 102 a and 102 b that aresymmetrical about a point to each other. That is, the crystal substrate101 has the concavities 102 a and 102 b formed therein by etching arectangular flat-shaped substrate body through masks (resist) 160covering its both major surfaces in such a manner that the bottom panelcommon to the concavities 102 a and 102 b form an ultrathin resonatingportion 103 integrally with the thick marginal portion 104. Due to adifference in etching rate between the z- and x-axis directions, theinner walls 104 a and 104 b of two sides of the annular marginal portion104 lying in the z-axis direction slope more gently than the other innerwalls lying in the x-axis direction. In addition, the both inner walls104 a and 104 b differ in inclination angle.

[0031] In etching, however, when the masks 160 having the apertures ofthe same shape are mounted on both sides of the crystal substrate 101 inalignment with each other, the z-axis side inner walls 104 a and 104 bof the concavities 102 a and 102 b bear such symmetric positionalrelationship as shown, in consequent of which edges 102′ and 102 b′ ofthe bottom surfaces of the respective concavities 102 a and 102 b arenot in aligned relation. Since the edges 102 a′ and 102 b′ of the bottomsurfaces of the concavities 102 a and 102 b are thus displaced in thez-axis direction relative to each other, the bottom surfaces of the twoconcavities are not directly opposite, decreasing the area of eachresonating portion 103 and consequently the effective thin region (theeffective vibrating region). This raises a problem of deterioratedcharacteristics of the crystal resonating elements with electrodes andso forth formed on such crystal substrates. In particular, furtherminiaturization of the piezoelectric substrate will increase theseverity of such a glitch.

[0032] [Fifth Prior Art Example]

[0033] A description will be given of, with reference to FIGS. 20 and21, of a conventional method for manufacturing the crystal resonatingelement with an ultrathin resonating portion. This is the manufacturingmethod that the inventor of present invention disclosed in TechnicalReport of the Institute of Electronics, Information and CommunicationEngineers of Japan, “UHF-Band Crystal Resonator Using Fundamental Wave,”(Technical Report of IEICE US98-27, EMD98-19, CPM98-51, OME98-49(1998-07), Corporation-Institute of Electronics, Information andCommunication Engineers of Japan).

[0034]FIG. 20 is a flowchart of a crystal resonator manufacturingprocess, and FIGS. 21(a) to (d) are longitudinal-sectional views showingthe crystal resonator in an etching process, the chain double-dashedlines X in FIGS. 21(a) to (d) being imaginary lines indicating thethickness of the resonating portion at the end of four stages of thechemical etching process.

[0035] In a crystal resonator for fundamental wave vibration in the UHFor higher band, for instance, since the amount of change in frequencywith respect to the amount of change in wafer thickness is large, thethickness of the crystal wafer is adjusted by four-stage chemicaletching steps 203 through 206 to obtain the resonating portion 103 thatis excitable at the desired fundamental-wave resonance frequency.

[0036] The manufacturing process begins with polishing the major surfaceof the crystal wafer (step 200), followed by vacuum depositing agold/chromium film on the polished major surface (step 201). In view ofa tradeoff between the mechanical strength of the wafer and the amountof etching, let it be assumed that the crystal wafer is 80 micrometers(m) thick. The gold/chromium film is selectively removed byphotolithography to form a mask pattern for etching (step 202).

[0037] Thereafter, first main etching (step 203) through secondfine-adjustment etching (step 206) processes are performed as describedbelow. In the first main etching process (step 203), as depicted in FIG.21(a), a crystal wafer 221 with a mask pattern for etching 224 formedthereon is subjected to wet etching to etch away the regions of thewafer underlying apertures of the mask pattern to form concavities asresonating portions 222 a and 223 a that resonate in the VHF band, forexample, at 155 MHz. In practice, however, since the thicknesses of theresonating portions 222 a and 223 a differ due to wafer etching errorsor the like, the resonance frequencies of the resonating portions 222 aand 223 a are measured.

[0038] And, in the first fine-adjustment etching process (step 204)shown in FIG. 21(b), an etchant is added dropwise to the respectiveconcavities for different periods of time based on the measuredresonance frequencies, by the technique disclosed, for example, in Pat.Laid-Open Gazette No. 6-021740, by which the thicknesses of theresonating portions 222 a and 223 a are individually adjusted so thattheir resonance frequencies be come as desired. Moreover, as depicted inFIG. 21(c), in second main etching process (step 205) the wafer isfurther subjected to wet etching to form resonating portions 222 c and223 c each having a thickness of about 2.2 μm that corresponds to aresonance frequency in the desired UHF band, for instance, at 760.9 MHz.Then the resonance frequencies of the resonating portions are measuredagain, and in second fine-adjustment etching shown in FIG. 21(d) dryetching is carried out for each of the resonating portions 222 c and 223c based on their measured frequencies so that they resonate at desiredfrequencies. After this, gold/chromium is vacuum deposited all over bothmajor surfaces of the wafer (step 207), then electrode patterns areformed thereon (step 208), and the wafer is severed into the crystalresonating elements 100 (step 209). The crystal resonating elements 100are each mounted in a package, then connected thereto by bonding orbumps (step 210), and sealed therein after being subjected to finalfrequency adjustment (step 211, 212).

[0039] The second fine-adjustment etching process (step 206) 114 isperformed by dry etching of low etching rate for high-precisionindividual adjustment to obtain the thickness of approximately 2.2 μmwhich corresponds to the desired resonance frequency of 760.9 MHz.

[0040]FIG. 22 is a longitudinal section view showing the working of theresonating portion; when the afore-mentioned four-stage (step 203through step 206) chemical etching for forming a resonating portion 232d from only one direction (indicated by the arrow), that is, from thedirection of the opening of the concavity, the area of the resonatingportion 232 a on the side of the opening of the concavity graduallydecreases and the area of the resonating portion 232 d becomes extremelysmall due to the dependence of the etching rate on the crystalorientation, and consequently, a vibrating region 232 h of theresonating portion 232 d becomes extremely narrow than a desired value.For example, in the case of obtaining a resonator whose fundamentalfrequency is 760.9 MHz, the thickness of the resonating portion is about2.2 μm, and if the thickness of the crystal wafer is set at 80 μm fromthe viewpoint of its mechanical strength, then it is etched to a depthof around 77.8 μm to form a concavity. Then, even if the opening of theconcavity is 0.7×0.55 millimeters (mm), the area of the vibrating region222 h is approximately 0.25×0.15 mm that is smaller than the desiredarea. For example, when the oscillation frequency is 622.3 MHz, thedesired size of the vibrating region 222 h is required to be in therange of 0.5 to 0.75×0.3 to 0.45 mm that is twice to three times (takinginto account variations caused during manufacturing process) larger thanthe size of the electrode to be formed in the vibrating region 232 h (anellipse of a size measuring a longer diameter 0.25×a shorter diameter0.15 mm).

[0041] Furthermore, the slope between the top surface of the annularmarginal portion 232 b and the vibrating region 232 h becomes so widethat a lead (not shown) formed on the slope becomes long, giving rise toa problem that the resistance or parasitic impedance of the leadincreases.

[0042] In the second main etching process (step 205) the etchant used isa low-temperature ammonium hydrogen fluoride saturated solution, whichprevents overetching but is low in working efficiency because of lowetching rate; since the second fine-adjustment etching (step 206) isperformed by dry etching of low etching rate for implementinghigh-precision adjustment, there is a problem of etching damage bycrystal defects, contamination with an impurity, or the like.

[0043] Furthermore, supply control (flow rate and pressure) of anetching gas for dry etching has so high a correlation with uniformity ofetching that the number of etching gas supply holes and their size mustbe changed for each operation; hence, it is difficult to obtain theoptimum conditions for etching.

[0044] Besides, since the etching process shown in FIG. 20 is followedby the gold/chromium vapor deposition step 207 for vapor depositing theconductive film that will ultimately form a main electrode film 110 andthen by the final frequency adjustment process (step 211) for makinghigh-precision frequency adjustments by vapor deposition or sputtering,the thickness adjustment by the etching process needs only to makeadjustments to such an extent as to allow compensation in the finalfrequency adjustment process (step 211), and hence the etching scheme inthis process is more than required.

[0045] Furthermore, the combined use of wet etching and dry etchinginevitably leads to complication of the manufacturing process and anincrease in capital investment, constituting an obstacle to bringingdown costs of UHF-band crystal resonators.

SUMMARY OF THE INVENTION

[0046] The present invention is intended to solve the above-mentionedproblems of the prior art; a first object of the invention, which copeswith the problem of the first prior art example, lies in that in thecase where ultraminiature piezoelectric substrates each having aresonating portion formed by a concavity made by etching in the surfaceof an anisotropic piezoelectric crystal material are mass-produced bybatch production using a large-area piezoelectric substrate wafer, theannular marginal portion surrounding the concavity is formedsufficiently thick to prevent individual piezoelectric substrates fromcracking when the wafer is severed thereinto. Another subject is toprevent a break in a conductive trace by routing it along the inner wallof the annular marginal portion, not on the steep slope formed thereonby a portion left unetched. Still another subject lies in that in thecase where a piezoelectric resonating element is supported in acantilever fashion in a package, the resonating portion is spaced as farapart from the cantilever supporting portion as possible to preventstress by the weight of the crystal resonating element from beingapplied to the resonating portion. Thus, the first subject is toimplement the optimum ultraminiature configuration of the piezoelectricsubstrate provided with an ultrathin resonating portion and a thickannular marginal portion surrounding it.

[0047] A second object of the invention, which copes with the problem ofthe second prior art example, lies in that the through holes (concavenotches), which are made by chemical etching as electrical connectingmeans for forming two connecting pads on either surface of eachsubstrate blank of a piezoelectric substrate wafer having piezoelectricsubstrates blanks arranged in sheet form, are prevented from inadequateformation due to limitations on the size of openings of the throughholes and reduction in productivity resulting from such inadequateformation of the through holes is also prevented.

[0048] A third object of the invention, which copes with the problem ofthe third prior art example, lies in that to obviate various glitcheswhich are caused when the thicknesses of resonating portions ofindividual concavities are each adjusted by etching for a differentperiod of time after the concavities are formed by batch production inthe piezoelectric substrate wafer, fine adjustments to the resonatingportions by etching the wafer from the side of its flat surface insteadof filling each concavity with the etchant.

[0049] A fourth object of the invention, which copes with the problem ofthe fourth prior art example, is to obviate the problem that in thepiezoelectric substrate having a thin resonating portion formed bymaking concavities by chemical etching in both major surfaces of thesubstrate made of an anisotropic crystal material, the effective area ofthe resonating portion is made small by a displacement of the opposedconcavities relative to each other in the one crystal orientation.

[0050] A fifth object of the invention, which copes with the problem ofthe fifth prior art example, is to provide a method for the manufactureof a high-efficiency but low-cost piezoelectric resonating element and,in particular, a method for the manufacture of a UHF-band AT-cut crystalresonator.

[0051] To attain the above objective, the piezoelectric substraterecited in claim 1 is a piezoelectric substrate made of an anisotropicpiezoelectric crystal material and provided with a thin resonatingportion and a thick annular portion integrally surrounding the outermarginal edge of said resonating portion, and a concavity formed in atleast one of major surfaces of said piezoelectric substrate; thepiezoelectric substrate is characterized in that the inner wall of saidannular portion in the one crystal orientation slopes at an angle lessthan that of the inner wall in the other crystal orientationperpendicular to said one crystal orientation, and that saidpiezoelectric substrate is longer in said one crystal orientation thanin said other crystal orientation.

[0052] In the case of a flat-shaped piezoelectric substrate made of ananisotropic piezoelectric material along two crystal axes crossing atright angles, when a concavity is formed by chemical etching in thepiezoelectric substrate surface, since the etching rate is higher in theone crystal orientation than in the other crystal orientation, the innerwall of an annular portion surrounding the concavity in said othercrystal orientation slopes gently (that is, forms a gentle slope). Inthe present invention, since one side of the annular portion having sucha gently sloping inner wall is extended to form a jut-out portion,dicing grooves or the like defining each piezoelectric substrate and theconcavities can be spaced sufficiently far apart in the mass productionof piezoelectric substrates (piezoelectric resonating elements) by batchproduction using a large-area piezoelectric substrate wafer, andconsequently, the annular portion can be formed thick. Accordingly, nocracking occurs in the annular portion when the wafer is severed alongthe dicing grooves. This implements optimum ultraminiaturization of theconfiguration of the piezoelectric substrate having an ultrathinresonating portion and a thick annular portion surrounding it.

[0053] The piezoelectric substrate recited in claim 2 is made of anAT-cut crystal and provided with a thin resonating portion, a thickannular portion integrally surrounding the outer marginal edge of saidresonating portion, and a concavity formed in at least one of majorsurfaces of said piezoelectric substrate; the piezoelectric substrate ischaracterized in that the piezoelectric substrate made of the AT-cutcrystal is longer in a z′-axis direction than in an x-axis direction.

[0054] When the above-mentioned piezoelectric substrate is an AT-cutcrystal substrate, the substrate may preferably be made longer in thez-axis direction than in the x-axis direction.

[0055] The piezoelectric resonating element recited in claim 3 isprovided with excitation electrodes formed opposite on both sides ofsaid resonating portion of the piezoelectric substrate of claim 1 or 2,a lead electrode extending from each of said excitation electrodes toone marginal edge of the piezoelectric substrate lengthwise thereof anda connecting pad connected to the lead electrode; the piezoelectricresonating element is characterized in that the lead electrode extendingfrom the excitation electrode on the side of the concavity is routed outvia said gently sloped inner wall of the annular portion.

[0056] With such a structure, it is possible to prevent a break in thelead electrode (a conductor trace) by bypassing a steep slope formed byan unetched portion on the inner wall of the annular portion.

[0057] The piezoelectric resonator recited in claim 4 is characterizedin that the piezoelectric substrate forming the piezoelectric resonatingelement of claim 3 is held at one end in its lengthwise direction in acantilever fashion in a surface-mount package.

[0058] With such a structure, when the piezoelectric resonating elementis held in a cantilever fashion in the package, it is possible toprevent stress caused by the weight of the crystal resonating elementfrom being applied to the resonating portion by maximizing the distancefrom the cantilever holding portion to the resonating portion.

[0059] The surface-mount piezoelectric oscillator recited in claim 5 ischaracterized by the provision of at least the piezoelectric resonatorof claim 4 and an oscillation circuit.

[0060] The piezoelectric substrate recited in claim 6 is provided with athin resonating portion, a thick annular portion integrally surroundingthe outer marginal edge of said resonating portion, a concavity formedin at least one of major surfaces of said piezoelectric substrate, and ajut-out portion formed by extending one side of said annular potion; thepiezoelectric substrate is characterized in that said jut-out portionhas formed in its forward marginal edge at least one concave notch openinto either surface of said piezoelectric substrate.

[0061] In the case where connecting pads connected to two excitationelectrodes are formed on either surface of the jut-out portion of thepiezoelectric substrate, they can be connected to pads on the bottom ofthe package by single coating of a conductive adhesive-this eliminatesthe need for using a package with a large outer peripheral wall. On theother hand, in the case where two connecting pads on either surface ofsaid jut-out portion, either surface of the piezoelectric substrate canbe held up or down as desired when it is mounted in the package; hence,the two connecting pads disposed on both surfaces need to beinterconnected via two concave notches in the forward marginal edge ofthe jut-out portion. In this case, if the two concave notches arearranged in the width of the forward marginal edge of the jut-outportion as in the past, the width of each concave notch becomesextremely small, increasing the possibility that the concave notches(through holes) open to either surface of the substrate cannot be formedby etching the piezoelectric substrate wafer. In view of this, accordingto the present invention, elongated through holes lying astride adjacentsubstrates are formed through the wafer to eliminate the possibility ofinsufficient formation of the concave notches by poor etching.

[0062] The piezoelectric substrate recited in claim 7 is characterizedin that said concave notches are each formed at one of two corners ofthe forward marginal edge of said jut-out portion in claim 6.

[0063] It is effective to form said through holes at both corners of theforward marginal edge of the jut-out portion of each piezoelectricsubstrate of the piezoelectric substrate wafer so that the through holeslie astride adjacent substrate regions.

[0064] The piezoelectric resonating element recited in claim 8 isprovided with excitation electrodes formed opposite on both surfaces ofsaid resonating portion of the piezoelectric substrate of claim 6 or 7and lead electrodes extending from the respective excitation electrodesto the forward marginal edge of said hut-out portion; the piezoelectricresonating element is characterized in that the lead electrode on eitherone of the substrate surfaces is routed through said concave notch tothe other substrate surface and connected to the connecting pad formedthereon.

[0065] It is possible to form two connecting pads side by side on thesame surface of the jut-out portion, or two connecting pads side by sideon either surface of the jut-out portion.

[0066] The piezoelectric resonator recited in clam 9 is characterized inthat the two connecting pads formed side by side on the same surface ofthe jut-out portion of the piezoelectric substrate forming thepiezoelectric resonating element of claim 8 are fixedly connected by aconductive adhesive to pads in the surface-mount package, respectively.

[0067] The surface-mount piezoelectric oscillator recited in claim 10 ischaracterized by the provision of at least the piezoelectric resonatorof claim 9 and an oscillation circuit.

[0068] The piezoelectric substrate wafer recited in claim 11 is apiezoelectric substrate wafer having the piezoelectric substrates ofclaims 6 to 8 arranged in sheet form, which is characterized in thatsaid concave notches are formed by through holes simultaneously made inthe wafer astride adjacent piezoelectric substrates.

[0069] It is possible to make the through holes large which are formedastride adjacent substrate regions of the wafer. Accordingly, in thecase of forming small concave notches by simultaneously etching bothsurfaces of the piezoelectric substrate, the both small concave notchescan be made to communicate with each other to form the through holes.Alternatively, the number of concave notches in the edge face of onepiezoelectric substrate needs not to be two, but instead one elongatedhole may also suffice which is made in the forward marginal edge of thejut-out portion. In this instance, the elongate through hole needs onlyto be made within a certain width in the forward marginal edge of thejut-out portion of the piezoelectric substrate of the wafer.

[0070] The piezoelectric substrate wafer recited in claim 12 is apiezoelectric substrate wafer having the piezoelectric substrates ofclaims 6 or 7 arranged in sheet form, which is characterized in thatsaid concave notches are formed by through holes made astride unusedregions of adjacent substrates.

[0071] With the piezoelectric substrates arranged directly inside-by-side relation, in the case of measuring characteristics of eachpiezoelectric resonating element with a probe pin held in contact witheach of connecting pad after forming the excitation electrodes, the leadelectrodes and the connecting pads, the contact pressure of the probepin causes a change in the resonance frequency of the resonatingelement, making accurate measurement impossible. To avoid this, unusedregions (dummy regions) are provided between adjacent piezoelectricsubstrates and the connecting pads are each formed astride the adjoiningunused regions. When conducting measurement with the probe pin held incontact with the connecting pad on the unused region, no bad influenceis exerted on measured values by the contact pressure of the probe pin.In particular, the formation of dicing grooves in the substrate surfacebetween the piezoelectric substrates and the unused regions furtherlessens bad influence of the contact pressure.

[0072] The piezoelectric substrate recited in claim 13 is provided witha thin resonating portion, a thick annular portion integrallysurrounding the outer marginal edge of said resonating portion and aconcavity formed in at least one of major surfaces of the substrate; thepiezoelectric substrate is characterized in that a thicknessfine-adjustment portion for the resonating portion is provided on thesubstrate surface on the opposite side from said concavity.

[0073] Conventionally, plural concavities are formed by etching in oneof major surfaces of a piezoelectric wafer at predetermined intervals,after which an etchant is filled into each concavity through a guidemask mounted on the substrate surface so as to make fine-adjustment tothe thickness of the resonating portion in the concavity; but when theconcavity becomes ultraminiaturized, the etchant permeates to otherregions through the dicing grooves cut between the substrate regions andetches portions which need not be etched, causing glitches such asreduction in the mechanical strength of the substrate.

[0074] According to the present invention, the fine adjustment to thethickness of the resonating portion is made by filling an opening of theguide mask mounted on the side of the flat surface of the wafer with theetchant-this obviates the defects of the prior art and ensuresadjustment to the thickness of the resonating portion of thepiezoelectric substrate having an ultraminiaturized concavity.

[0075] The piezoelectric resonating element recited in claim 14 ischaracterized by the provision of the excitation electrodes formedopposite on both sides of said resonating portion of the piezoelectricsubstrate of claim 13, lead electrodes each extending from one of theexcitation electrodes to one end edge of the piezoelectric substratelengthwise thereof, and connecting pads each connected to one of thelead electrodes.

[0076] The piezoelectric resonator recited in claim 15 is characterizedin that the piezoelectric substrate forming the piezoelectric resonatingelement of claim 14 is fixedly held at one end to the inside of asurface-mount package in a cantilever fashion.

[0077] The piezoelectric oscillator recited in claim 16 is characterizedby the provision of at least the piezoelectric resonator of claim 15,and an oscillation circuit.

[0078] The piezoelectric substrate wafer recited in claim 17 ischaracterized in that a plurality of such piezoelectric substrates ofclaim 13 are arranged in a sheet form.

[0079] The piezoelectric substrate wafer recited in claim 18 ischaracterized in that adjacent piezoelectric substrates of saidpiezoelectric substrate wafer are separated by a dead space through twoparallel dicing grooves therebetween in claim 17.

[0080] With a dummy region as the dead space interposed between adjacentpiezoelectric substrates, it is possible to avert the bad influence bythe etchant during the thickness adjustment conducted from the side ofthe flat wafer surface.

[0081] The piezoelectric substrate wafer manufacturing method recited inclaim 19 is characterized in that the resonating portion thicknessfine-adjustment portion, formed on the piezoelectric substrate wafer ofclaim 17 or 18 in the wafer surface opposite from the concavity of eachpiezoelectric substrate, is formed by mounting a guide mask, which has aplurality of openings each larger than the concavity and arranged in agrid pattern, on the piezoelectric substrate wafer on said oppositesurface and then filling an etchant into each opening of said guidemask.

[0082] The piezoelectric substrate recited in claim 20 is made of ananisotropic piezoelectric crystal material and provided with a thinresonating portion, a thick annular portion integrally surrounding theouter marginal edge of said resonating portion and concavities formed onboth major surfaces of the piezoelectric substrate; the piezoelectricsubstrate is characterized in that said concavities are each configuredso that the inner wall of the annular portion in the one crystalorientation has a smaller inclination angle than does the inner wall ofthe annular portion in the other crystal orientation crossing said oneorientation at right angles thereto and that those marginal edges of theinner bottoms of said concavities lying in the same crystal orientationare aligned with each other.

[0083] In the case of forming the concavities in the piezoelectricsubstrate of an anisotropic piezoelectric crystal material by etchingfrom its both surfaces through masks of the same configuration, ifrespective openings of the both masks are aligned with each other, theboth concavities each have point symmetry and marginal edges of theinner bottoms of the both concavities (in particular, the marginal edgesin crystal orientation in which the etching rate is low) are not alignedwith each other. On this account, the area of the thin resonatingportion becomes small.

[0084] According to the present invention, the marginal edges of theopenings of the both masks (in particular, the marginal edges in thecrystal orientation in which the etching rate is low) are predisplacedas predetermined to bring the marginal edges of the both concavitiesinto alignment with each other after etching. This permits maximizationof the area of the resonating portion, making it possible to obtainhighly reliable piezoelectric substrates, piezoelectric resonators andso on.

[0085] The piezoelectric substrate recited in claim 21 is characterizedin that it is made of an AT-cut crystal in claim 20.

[0086] The piezoelectric resonating element recited in claim 22 ischaracterized by the provision of excitation electrodes formed oppositeon both surface of said resonating portion of the piezoelectricsubstrate of claim 20 or 21, lead electrodes each extending from one ofthe excitation electrodes to one marginal edge of the piezoelectricsubstrate in its lengthwise direction, and connecting pads connected tothe lead electrodes, respectively.

[0087] The piezoelectric resonator recited in claim 23 is characterizedin that the piezoelectric substrate forming the piezoelectric resonatingelement of claim 22 is fixed held at one end to the inside of asurface-mount package in a cantilever fashion.

[0088] The surface-mount piezoelectric oscillator recited in claim 24 ischaracterized by the provision of at least the piezoelectric resonatorof claim 23 and an oscillation circuit.

[0089] The piezoelectric substrate manufacturing method recited in claim25 is a method for the manufacture of a piezoelectric substrate made ofan anisotropic piezoelectric crystal material and provided with a thinresonating portion, a thick annular portion integrally surrounding theouter marginal edge of said resonating portion, and concavities in bothmajor substrate surfaces, the inner wall of the annular portion definingeach concavity in the one crystal orientation sloping at an angle lessthan that of the inner wall in the other crystal orientationperpendicular thereto, the method comprising the steps of: covering theboth major surfaces of the flat-shaped piezoelectric substrate withmasks for etching therethrough the substrates to form said concavities;and etching the piezoelectric substrate through said masks to form theconcavities in those areas of the both major surfaces of thepiezoelectric substrate which are exposed in the openings of the masks;characterized in that the position of each of said mask is shifted insaid one crystal orientation to bring the marginal edges of the bottomsurfaces of said concavities into alignment with each other.

[0090] The piezoelectric substrate manufacturing method recited in claim26 is characterized in that said piezoelectric substrate in claim 25 isa piezoelectric substrate wafer having a plurality of piezoelectricsubstrates arranged in sheet form.

[0091] The piezoelectric resonating element manufacturing method recitedin claim 27 is a method for the manufacture of a piezoelectricresonating element having a thin resonating portion formed by aconcavity made in one of major surfaces of a piezoelectric substrate,which method comprises a first main etching step of forming theresonating portion by etching away a predetermined portion of the onemajor surface, a frequency measuring step of measuring the resonancefrequency of said resonating portion, a first fine-adjustment etchingstep of making fine-adjustments to the thickness of said resonatingportion based on the frequency measured by said frequency measuringstep, and a second main etching steps of further reducing the thicknessof said resonating portion, and a second fine-adjustment etching step ofmaking fine-adjustments to the thickness of said resonating portion, themethod being characterized in that said etching steps are both performedby wet etching.

[0092] The piezoelectric resonating element manufacturing method recitedin claim 28 is characterized in that said second main etching step inclaim 27 is to perform etching over the entire area of the other majorsurface of said piezoelectric wafer.

[0093] The piezoelectric resonating element manufacturing method recitedin claim 29 is characterized in that said second main etching step inclaim 27 is to perform etching over the entire area of either majorsurface of said piezoelectric wafer in claim 27.

[0094] The piezoelectric resonating element manufacturing method recitedin claim 30 is characterized in that said second fine-adjustment etchingstep in any one of claims 27 to 29 is to perform etching over the entirearea of the other major surface of said piezoelectric wafer.

[0095] The piezoelectric resonating element manufacturing method recitedin claim 31 is characterized in that said second fine-adjustment etchingstep in any one of claims 27 to 29 is to perform etching over the entirearea of either major surface of said piezoelectric wafer.

[0096] The piezoelectric resonating element manufacturing method recitedin claim 32 is characterized in that the manufacturing method of any oneof claims 27 to 31 includes a step of forming a plurality of concavitiesin one piezoelectric wafer and dividing the wafer into a plurality ofpiezoelectric resonating elements.

[0097] The piezoelectric resonating element manufacturing method recitedin claim 33 is characterized in that said frequency measuring step inclaim 32 is to conduct frequency measurements for all of the resonatingportions and that said first fine-adjustment etching step is to performetching for each of the resonating portions.

[0098] The piezoelectric resonating element manufacturing method recitedin claim 34 is characterized in that said frequency measuring step inany one of claims 32 to 33 is to conduct frequency measurements for someof the plurality of resonating portions and that said second mainetching step and said second fine-adjustment etching step are to performetching simultaneously for all of the resonating portions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0099] FIGS. 1(a), (b) and (c) are perspective and plan views showing acrystal resonating element made of an AT-cut crystal as an example ofthe piezoelectric resonating element according to an embodiment of thepresent invention, and a diagram showing the principal part of a wafer.

[0100]FIG. 2 is a sectional view of a crystal resonator with the crystalresonating element of FIG. 1 hermetically sealed in a package.

[0101]FIG. 3 is a diagram showing, by way of example, the crystalresonating element of the present invention as being applied to asurface-mount crystal oscillator.

[0102] FIGS. 4(a), (b) and (c) are a perspective view of the crystalresonating element (crystal substrate) according to an embodiment of thepresent invention corresponding to the second prior art example, asectional view showing the crystal resonating element mounted in apackage, and a diagram explanatory of the wafer configuration.

[0103]FIG. 5 is a plan view showing the principal part configuration ofa piezoelectric substrate wafer according to another embodiment of thepresent invention.

[0104] FIGS. 6(a) and (b) are a diagram showing the principal partconfiguration of a piezoelectric substrate wafer according to anotherembodiment, and a perspective view of a crystal resonating elementblank.

[0105] FIGS. 7(a) and (b) are explanatory diagrams of an embodimentcorresponding to the third prior art example, and (c) is an explanatorydiagram of a thickness fine-adjustment portion.

[0106]FIG. 8(a) is a sectional view of a piezoelectric substrateaccording to an embodiment corresponding to the fourth prior artexample, and (b) is a sectional view of a piezoelectric resonator.

[0107]FIG. 9(a) is a perspective view showing, by way of example, acrystal resonating element made of an AT-cut crystal as thepiezoelectric resonating element according to the present invention, and(b) is a longitudinal-sectional view taken on the line A-A in FIG. 9(a).

[0108]FIG. 10 is a diagram for explaining a sequence of steps involvedin the manufacture of the crystal resonator according to an embodimentof the present invention.

[0109]FIG. 11(a) is a longitudinal-sectional view of the crystalresonator at the end of first fine-adjustment etching during etchingprocess in the embodiment of the present invention, and (b) is alongitudinal-sectional view of the crystal resonator at the end ofsecond fine-adjustment etching.

[0110] FIGS. 12(a) to (d) are longitudinal-sectional views showing asequence of steps of etching in the piezoelectric resonatormanufacturing method according to the present invention.

[0111]FIG. 13 is a table showing conditions for etching in themanufacturing method according to the present invention.

[0112] FIGS. 14(a) to (d) are diagrams for explaining the direction ofetching in the embodiment of the present invention.

[0113] FIGS. 15(a) and (b) perspective and sectional views showing theconfiguration of an AT-cut crystal resonating element as an example of aconventional piezoelectric resonating element, and (c) is a sectionalview showing the crystal resonating element mounted in a surface-mountpackage.

[0114]FIG. 16 is a diagram showing the principal part configuration of apiezoelectric substrate wafer used to form the piezoelectric substrateof FIG. 15.

[0115] FIGS. 17(a) and (b) are a sectional view of a surface-mountcrystal resonator according to another prior art example and a sectionalview taken on the line A-A in FIG. 17(a), (c) is a perspective viewshowing concave notches made in a forward marginal edge of a jut-outportion, and (d) is a diagram showing the principal part configurationof the piezoelectric wafer used.

[0116] FIGS. 18(a), (b) and (c) are diagrams for explaining aconventional method for fine-adjustment for each concavity, and (d) is adiagram for explaining a defect of the conventional fine-adjustmentmethod.

[0117]FIG. 19 is a sectional view of a conventional piezoelectricsubstrate having concavities in both surfaces thereof.

[0118]FIG. 20 is a diagram for explaining a conventional crystalresonator manufacturing process.

[0119] FIGS. 21(a) to (d) are longitudinal-sectional views of thecrystal resonator in respective etching steps in the conventionalmanufacturing method.

[0120]FIG. 22 is a longitudinal-sectional view showing a resonatingportion formed by the conventional manufacturing method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0121] The present invention will hereinafter be described in detailwith reference to its embodiments shown in the drawings.

[0122] [Embodiment Corresponding to First Prior Art Example]

[0123] FIGS. 1(a) and (b) are perspective view and plan views showing acrystal resonating element 1 made of an AT-cut crystal as an example ofthe piezoelectric resonating element according to an embodiment of thepresent invention.

[0124] The crystal resonating element 1 is provided with a crystalsubstrate 2 made of an AT-cut crystal as an anisotropic piezoelectricmaterial, excitation electrodes formed on both major surfaces of thecrystal substrate 2, respectively, lead electrodes 11 a and 11 bextending from the excitation electrodes 10 a and 10 b, respectively,and connecting pads 12 a and 12 b forming respective lead electrode endportions.

[0125] The crystal substrate 2 has a construction in which an ultrathinresonating portion 4 is formed by the bottom of a concavity 3 formed byetching in one of major surfaces of a rectangular flat-shaped substratelonger in the z-axis direction and the outer marginal edge of theresonating portion 4 is integrally held by a thick annular portion 5.One side 5A of the annular portion 5 lying in the z-axis direction isextended a predetermined length in the z-axis direction to form aflat-shaped jut-out portion 6. On one side of the jut-out portion 6there are routed the lead electrodes 11 a and 11 b extending from theexcitation electrodes and the connecting pads 12 a and 12 b are disposedat the ends of the lead electrodes 11 a and 11 b.

[0126] On characteristic point in which the crystal resonating element 1according to this embodiment differs from the crystal resonating elementof the prior art example is that the crystal substrate 2 takes a shapeof a rectangle longer in the z-axis so that the annular portion 5 issufficiently wide and mechanically strong. Accordingly, the gentlestslope 5 a is located on the side of the jut-out portion 6. And the leadelectrode 11 a extended from the excitation electrode 10 a disposed onthe bottom of the concavity forming the resonating portion 4 can berouted along the gentlest slope 5 a. Furthermore, when the crystalresonating element is supported in a cantilever fashion in a package,the supporting point and the resonating portion can be spaced as farapart as possible.

[0127] When the concavity 3 is formed in the major surface of thecrystal substrate 2 by an etching operation using a required etchant,the substrate surface is covered with a mask through which to expose thesurface area corresponding to the concavity 3; in this case, of the fourinner walls of the concavity 3, the inner walls 5 a and 5 b in thez-axis direction form gentler slopes as unetched portions than the innerwalls in the x-axis direction. This embodiment takes advantage of thisphenomenon in etching to lay out the crystal substrate 2 so that thejut-out portion 6 is located in the z-axis direction of the concavity 3.Accordingly, when crystal substrates are arranged on a wafer 30 withdicing or dividing grooves 31 formed between them, they are longsideways as depicted in FIG. 1(c). In this instance, the area of eachcrystal substrate is the same as is the case with the conventionalcrystal substrate shown in FIG. 16.

[0128] That is, according to the present invention, the area and shapeof the crystal substrate itself are the same as in the FIG. 16 prior artexample, but the lengthwise direction of the crystal substrate is thez-axis direction. Accordingly, the spacing w between the three marginaledges of the concavity 3 and the respective dividing grooves 31 can bemade sufficiently large to provided increased thickness of the annularportion 5, and hence it is possible to prevent the occurrence ofcracking when the wafer is cut along the cutting roves 31.

[0129] Further, too close spacings between the lead electrodes 11 a and11 b and between the connecting pads 12 a and 12 b are likely to causeelectrical interference and exert bad influence on characteristics ofthe crystal resonating element; accordingly, when the lead electrode 11a on the concavity side is formed along the gentle slope 5 a, the leadelectrode 11 b from the flat surface side is extended to the sidesurface of the concavity along a route spaced as far apart from the leadelectrode 11 a as possible.

[0130] The connecting pad 12 a connected to the lead electrode 11 a onthe concavity side is disposed at the one corner of the jut-out portion6 in its widthwise direction (in the x-axis direction), whereas the leadelectrode 11 b routed from the flat surface side to the side surface ofthe concavity is disposed at the other corner to space the connectingpad 12 b as far apart from the connecting pad 12 a.

[0131]FIG. 2 is a sectional view showing the state in which the crystalresonating element 1 of FIG. 1 is mounted in a surface-mount package 20and hermetically sealed therein. The package 20 is provided with apackage body 21 having a open-topped cavity, and a metal cover 26 forclosing the opening of the cavity of the package body 21. The packagebody 21 has an external electrode 22 formed on the underside of thebody, and an internal electrode 23 formed on the bottom of the cavityand connected to the external electrode 22, and support the crystalresonating element 1 in a cantilever fashion by electrically andmechanically connecting the connecting pads 12 to the internal electrode23 through a conductive adhesive 25.

[0132] In this case, the connecting portion (supporting portion) betweenthe connecting pads 12 and the internal electrode 23 and the resonatingportion 4 are interconnected through the gentlest slope 5 a among theinner walls of the concavity 3. That is, the gentle slope 5 a isutilized to space the connecting portion by the adhesive far apart fromthe resonating portion 4, and the cantilever structure relieves thestress on the resonating portion to prevent it from distortion.

[0133]FIG. 3 shows an example of application of the crystal resonatingelement 1 of the present invention to a surface-mount crystaloscillator; the crystal oscillator 40 has a construction in which, forexample, the crystal resonating element 1 is supported in a cantileverfashion by fixing the connecting pad 12 on the internal electrode 23formed on a stepped portion in the package body 21 by the conductiveadhesive 25, circuit pats 41 forming an oscillation circuit and so onare mounted on pads formed on the bottom of the package body 21 and thecavity of the package body 21 is sealed with the metal cover 26.

[0134] Incidentally, while the above embodiment exemplifies the AT-cutcrystal as an anisotropic piezoelectric crystal material, it is merelyillustrative and the invention is applicable to every anisotropicpiezoelectric crystal material (This goes for other embodiments). Thatis, the piezoelectric substrate configuration of the present inventionis applicable to a piezoelectric substrate made of an anisotropicpiezoelectric crystal material and having a thin resonating portion anda thick annular portion integrally surrounding the outer marginal edgeof the resonating portion to thereby form a concavity in at least one ofmajor substrate surfaces; in this instance, the outer dimensions of thepiezoelectric substrate are set such that the length of the substrate inthe direction in which the gentle slope of the smallest inclinationangle among those of the inner walls of the concavity exists is longerthan in the direction perpendicular to that in which the gentle slopeexists.

[0135] With such a structure as described above, it is possible toimplement formation of a thick annular portion, prevention of breaks inthe lead electrodes, and prevention of the occurrence of distortion orstress in the resonating portion when it is mounted in a cantileverfashion in the package.

[0136] [Embodiment Corresponding to Second Prior Art Example]

[0137] FIGS. 4(a), (b) and (c) are a perspective view of a crystalresonating element (a crystal substrate) according to an embodiment ofthe present invention corresponding to the second prior art example, asectional view of the resonating element mounted in a package, and adiagram explanatory of the wafer configuration. Incidentally, thisembodiment shows an example of using the crystal substrate as apiezoelectric material, but it is intended as being merely illustrativeand the present invention is applicable to any piezoelectric materials.

[0138] The crystal resonating element 1 is provided with a crystalsubstrate 2 made of an AT-cut crystal used as a piezoelectric crystalmaterial, excitation electrodes 10 a and 10 b formed on both majorsurfaces of the crystal substrate 2, respectively, lead electrodes 11 aand 11 b extending from the excitation electrodes 10 a and 10 b,respectively, and connecting pads 12 a, 12 a′ and 12 b, 12 b′ formingend portions of the lead electrodes.

[0139] The crystal substrate 2 is a piezoelectric substrate of theconfiguration that has a thin resonating portion 4 and a thick annularportion 5 integrally surrounding the outer marginal edge of theresonating portion to form a concavity 3 in at least one of the majorsurfaces of the substrate, and it is provided with a jut-out portion 6formed by extending one side 5A of the annular portion 5. The forwardmarginal edge 6 a of the jut-out portion 6 has at least one pair ofconcave notches 7 a and 7 b open to the both surfaces of the crystalsubstrate 2. In this example, the concave notches 7 a and 7 b areprovided at both end portions of the forward marginal edge 6 a, that is,at both corners of the jut-out portion 6, and the inner walls of theconcave notches 7 a and 7 b are coated with conductive films connectedto the connecting pads 12 a, 12 a′ and 12 b, 12 b′.

[0140] The connecting pad 12 connected to the lead electrode 11 aextending from the excitation electrode 10 a disposed on the concavity 3side is connected to the connecting pad 12 a′ on the flat surface of thesubstrate via the conductive film coated on the inner wall of theconcave notch 7 a. On the other hand, the connecting pad 12 b connectedto the lead electrode 11 b extending from the excitation electrode 10 bdisposed on the flat surface of the substrate is connected to theconnecting pad 12 b′ on the concavity side via the conductive filmcoated on the inner wall of the concave notch 7 b.

[0141] Incidentally, it is also possible to form only one concave notch,through which only one of the lead electrodes is routed to the oppositesubstrate surface for connection to a second connecting pad formedthereon.

[0142] In this embodiment, as described above, either of the leadelectrodes 11 a and 11 b extending from the excitation electrodes 10 aand 10 b formed opposite on both sides of the resonating portion 4 ofthe piezoelectric substrate 2 to the forward marginal edge 6 a of thejut-out portion 6 is routed to the opposite substrate surface via theconductive film coated on the inner wall of the concave notch andconnected to the connecting pad on the said opposite substrate surface;hence, when the crystal resonating element 1 is mounted in the package20 with the concavity held downward as shown in FIG. 4(b), the twoconnecting pads 12 a and 12′ position themselves on the jut-out portion6 on the side of the concavity and can be connected to connecting pads23 a and 23 b of the package side by giving a single coating of aconductive adhesive onto the connecting pads of the resonatingelement—such a structure precludes the possibility of the conductiveadhesive protruding toward the flat surface of the substrate. Thiseliminates the need for increasing the height of the package 20according to the amount of conductive adhesive protruding, and hencepermits reduction in profile of the package.

[0143] Moreover, the provision of the pairs of connecting pads 12 a, 12a′ and 12 b, 12 b′ on the both sides of the jut-out portion 6 allows afree choice of the orientation of the crystal resonating element 1 whenit is mounted in the package.

[0144] Next, a description will be given, with reference to FIG. 4(c),of the procedure for mass-producing the piezoelectric substrate 2 orcrystal resonating element 1 of the present invention by a batchoperation using a large-area crystal substrate wafer (a piezoelectricsubstrate wafer) 30. (Incidentally, only one substrate in FIG. 4(c) isshown to have formed thereon a conductive pattern, for reference sake.)That is, in this embodiment, rectangular spaces defined by rows andcolumns of dividing grooves 31 formed in the wafer surface are used asindividual crystal substrates, and the concavity 3 and through holes 7Hforming the concave notches 7 a and 7 b are formed by chemical etchingusing a required etchant and a mask. A structural feature of this waferlies in that the through hole 7H forming the concave notches is formedastride two laterally adjacent substrates so that the concave notches 7a and 7 b are positioned at both corners of the forward marginal edge 6a of the jut-out portion 6.

[0145] Incidentally, the through hole 7H is obtained by joining togethera pair of directly opposed small concave notches formed in bothsubstrate surfaces by simultaneous etching as referred to previously inrespect of the prior art. In this case, there is the possibility thatthe small concave notches cannot completely be interconnected if theirdiameters are too small.

[0146] As described above, in this embodiment the concave notches 7 aand 7 b are simultaneously formed in adjacent crystal substrates bymaking the through hole 7H astride them in the wafer 30. In thisinstance, the through hole 7H is formed by two neighboring concavenotches, and hence it has a large size accordingly; thus, bysimultaneous formation of the small concave notches by etching in theboth substrate surfaces at the corresponding position, it is possible tosharply reduce the possibility that the both small concave notches arenot completely joined together, resulting in a failure in the formationof the through hole.

[0147] Thereafter, the excitation electrodes 10 a, 10 b, the leadelectrodes 11 a, 11 b, and the connecting pads 12 a, 12 a′ and 12 b, 12b′ are respectively formed on both sides of each substrate by anarbitrary method such as vapor deposition or sputtering using a requiredmask, and at the same time the inside surfaces of the concave notches 7a and 7 b are each coated with a conductive film.

[0148] After the formation of these conductor traces, characteristics ofeach crystal resonating element, such as its resonance frequency and soon, are measured with probe pins of measuring equipment held on theconnecting pads 12 a′, 12 b on the flat surface side, or on theconnecting pads 12 a and 12 b′ on the concavity side, then based on themeasured results, a thickness adjustment operation is performed for eachsubstrate, followed by severing the wafer along the dividing grooves 31.

[0149] By the way, after each crystal resonating element is formed bydepositing the excitation electrodes and other conductor traces on eachcrystal substrate on the crystal substrate wafer 30, the characteristicsof each crystal resonating element are measured using probe pins; inthis case, if the substrates directly adjoin as depicted in FIG. 4(c),the connection of the probe pins to the two connecting pads 12 a and 12b′ on one crystal resonating element under measurement is made insidethe right and left dividing grooves 31. But, since the distance betweenthe position where to connect the probe pins to the connecting pads andthe resonating portion 4 is as short as less than 0.5 mm, even theslightest pressure of the probe pins on the substrate surface throughthe connecting pads is likely to affect the resonance frequency of theresonating portion 4, constituting a factor making it difficult toachieve accurate measurement.

[0150]FIG. 5 is a plan view showing the principal part of apiezoelectric substrate wafer according to an embodiment configured toobviate such a problem.

[0151] A structural feature of this piezoelectric substrate wafer 30lies in that the crystal resonating element 1 (the piezoelectricsubstrate 2) is flanked on either side by a dead space 50 as a regionwhere no substrate is formed, with the resonating element and the deadspace separated by the dividing line 31, and that there are formed onthe dead space 50 dummy connecting pads 51 for connection with theconnecting pads 12 a, 12 b′ or 12 a′, 12 b on the crystal resonatingelement. The dummy connecting pads 51 are connected to the connectingpads on the both surfaces of the adjoining substrates through theconductive films coated on the inside surfaces of the through holes 7H(concave notches 7 a, 7 b).

[0152] In the case of measuring the characteristics of each resonatingelement 1 on the piezoelectric substrate wafer of such a configurationas described above, probe pins of non-shown measurement equipment arenot directly held in contact with the connecting pads 12 a, 12 b′ or 12a′, 12 b, but instead they are contacted with the dummy connecting pads51 disposed adjacent the connecting pads separated therefrom by thedividing grooves 31.

[0153] In this case, the transfer of stresses caused by contacting theprobe pins with the dummy connecting pads 51 is interrupted by thedividing grooves 31—this lessens the influence of the stresses on theresonating portion 4 of the crystal resonating element 1, permittingaccurate measurement.

[0154] Incidentally, while in the illustrated example each dead space 50is shown to be of the same area as that of the adjoining piezoelectricsubstrate 2, this is illustrative only and the areas of the dead space50 may be further decreased.

[0155] In the embodiments of FIGS. 4 and 5, since the through hole 7H ismade astride adjacent substrate regions, or the substrate region and theadjoining dead space, two concave notches 7 a and 7 b are each formed atone end portion of the forward marginal edge 6 a of the jut-out portion6 of one piezoelectric substrate 2, but only one concave notch alsosuffices.

[0156] FIGS. 6(a) and (b) are a diagram showing the principal part of apiezoelectric wafer according to another embodiment of the presentinvention and a perspective view of a crystal resonating element. Thepiezoelectric substrate wafer 30 has elongated through holes 7H eachmade along the forward marginal edge 6 a of the jut-out portion 6 ofeach piezoelectric substrate 2, and separate conductor films 7C areformed on the inner wall of the through hole 7H to establish electricalconnections between the connecting pads 12 a, 12 a′, and 12 b, 12 b′, onboth side of the substrate.

[0157] The excitation electrodes 10 a, 10 b, the lead electrodes 11 a,11 b, the connecting pads 12 a, 12 a′, 12 b, 12 b′ and the separateconductor films 7C are formed, then frequency measurement is conductedfor each substrate with probe pins held in contact with the connectingpads or dummy connecting pads 51 (see FIG. 5), and the wafer is severedalong the dividing grooves 31 into individual crystal resonatingelements such as shown in FIG. 6(b). In this case, severing along thedividing grooves 31 renders each through hole 7H into the concave notch7. The separate conductor films 7C on the concave notch 7 are spacedapart from each other.

[0158] This embodiment needs only to make one through hole for eachsubstrate, and hence it simplifies the configuration of the mask foretching, cutting manufacturing costs and increasing mass-productivity.

[0159] Incidentally, excitation electrodes are formed opposite on bothsides of the resonating portion of the piezoelectric substrateconfigured as described above, and lead electrodes extending from theexcitation electrodes to the forward marginal edge of the jut-outportion and connecting pads are formed, by which a piezoelectricresonating element is obtained.

[0160] Then, two connecting pads disposed side by side on the same sideof the jut-out portion of the piezoelectric substrate forming theabove-mentioned piezoelectric resonating element are bonded to pads in asurface-mount package by use of a conductive adhesive, by which apiezoelectric resonator is obtained.

[0161] And, circuit parts forming an oscillation circuit areincorporated into the package forming such a piezoelectric resonator, bywhich a surface-mount piezoelectric oscillator is obtained.

[0162] [Embodiment Corresponding to Third Prior Art Example]

[0163] FIGS. 7(a) and (b) are diagrams explanatory of the embodimentcorresponding to the third prior art example; in this embodiment, aguide mask 60 having apertures 61 (partitions 62) of preset size andpitch is mounted on the flat surface side of the piezoelectric substratewafer 30, and then those flat surface regions of the vibrating portion 4of the piezoelectric substrate wafer 30 exposed in the apertures 61 areindividually etched.

[0164] That is, as referred to previously in respect of the prior art,due to technical limitations on machining, the minimization of the sizeand pitch of the apertures 61 of the guide mask 60 is limited. On thisaccount, it is inevitable to use the guide mask 60 for individuallyadjusting the thickness of the resonating portion 4 in the concavity 3smaller than the apertures 61 of the minimum size. However, theindividual etching with the guide mask 60 mounted on the concavity sideas in the past has too many demerits.

[0165] In view of the above, the present invention presets the spacingof the piezoelectric substrates 2 on the wafer 30 at a large valueaccording to the size and pitch of the apertures 61 of the guide mask60, and cuts two parallel dividing grooves 31 in the surface of thethick portions between the piezoelectric substrates 2. The thickness ofthe resonating portion 4 in each concavity 3 is premeasured, and thetime for adjusting the resonating portions 4 of different thicknesses toa uniform thickness is precalculated.

[0166] And, the guide mask 60 is fixedly mounted on the flat surfaceside of the wafer 30 so that the resonating portions 4 are positioned atthe centers of all the apertures 61 of the guide mask 60. Then, anetchant is filled into the concavities through the apertures 61 in apredetermined sequence. In this case, the etchant is filled into theconcavities in descending order of thickness of the resonating portion4, and at the time the thickness of every resonating portion has beenreduced to a predetermined value, all the substrates are simultaneouslycleaned out to remove the etchant.

[0167] As a result, a thickness fine-adjustment portion 65 of an areawider than that of the concavity is formed as a minute depression in theflat wafer surface exposed in the aperture 61 as shown in FIG. 7(c).Reference numeral 50 denotes a dummy region disposed between substrateregions.

[0168] As described above, in this embodiment, the guide mask is mountedon the flat surface side of the wafer, then an etchant is filed intoconcavities requiring fine-adjustment to the thickness of the resonatingportion for different periods of time, and after etching, the substratesare cleaned to remove the etchant by one operation; hence, the concavityside of the wafer is not adversely affected by the etchant and theetchant does not permeate other regions through the dividing grooves 31to cause glitches. Further, there is no possibility that the surfacetension of the etchant filled in the concavity prevents its closecontact with the inner wall of the concavity, causing insufficientetching and other glitches.

[0169] Incidentally, the thick portion defined by the two paralleldividing grooves 31 between the piezoelectric substrates is a deadspace. By sufficiently widening the width of the dead space, it ispossible to eliminate the possibility that the etchant filled into acertain aperture 61 exerting bad influence on adjoining piezoelectricsubstrate regions. By severing the wafer 30 along the dividing grooves31 after making such thickness fine-adjustments for the resonatingportion 4 in each concavity, it is possible to obtain individualpiezoelectric substrates 2 each having a thickness fine-adjustmentportion 65 for the resonating portion 4 on the substrate surface on theopposite side from the concavity 3.

[0170] Excitation electrodes are each formed on either of the resonatingportion 4 of such a piezoelectric substrate 2 in directly opposedrelation, and lead electrodes extending from the respective excitationelectrodes to one marginal edge of the piezoelectric substratelengthwise thereof and the connecting pads connected to the leadelectrodes are formed as by vapor deposition, by which a piezoelectricresonating element is obtained.

[0171] Then, the piezoelectric substrate forming such a piezoelectricresonating element is supported at one end in a surface-mount package ina cantilever fashion and the package is hermetically sealed with acover, by which a piezoelectric resonator is obtained.

[0172] And, circuit parts forming an oscillation circuit are integrallyassembled in place with the package forming the piezoelectric resonator,by which a surface-mount piezoelectric oscillator is obtained.

[0173] [Embodiment Corresponding to Fourth Prior Art Example]

[0174]FIG. 8(a) is a sectional view of a piezoelectric substrateaccording to an embodiment of the present invention corresponding to thefourth prior art example.

[0175] The piezoelectric substrate of this embodiment is a piezoelectricsubstrate made of an AT-cut crystal that is an example of an anisotropicpiezoelectric crystal material.

[0176] This crystal substrate 2 is provided with a thin resonatingportion 4 and a thick annular portion 5 integrally surrounding the outermarginal edge of the resonating portion 4 to form concavities 3 a and 3b in both major surfaces of the substrate. The inner walls 5 a and 5 bof each of the concavities 3 a and 3 b in the one crystal orientation(the z-axis direction) form gentler slopes than in the other crystalorientation (the x-axis direction) perpendicular thereto. And, edges 3a′ and 3 b′ of the both bottoms of the concavities 3 a and 3 b arealigned with each other. This permits maximization of the effective areaof the resonating portion 4.

[0177] In the case of forming the concavities 3 a and 3 b of such acrystal substrate 2 by chemical etching, the positions of those of themarginal edges of apertures 70 a′ and 70 b′ of masks (resist) 70 a and70 b covering the both sides of the substrate which lie in the z-axisdirection are predisplaced a predetermined distance L as depicted inFIG. 8(b). The predetermined distance L may be set such that themarginal edges 3 a′ and 3 b′, of the bottoms of the concavities 3 a and3 b are brought into alignment with each other when etching is carriedout.

[0178] Incidentally, excitation electrodes are each formed on either ofthe resonating portion 4 of such a piezoelectric substrate 2 in directlyopposed relation, and lead electrodes extending from the respectiveexcitation electrodes to one marginal edge of the piezoelectricsubstrate lengthwise thereof and the connecting pads connected to thelead electrodes are formed, by which a piezoelectric resonating elementis obtained.

[0179] Then, the piezoelectric substrate 2 forming such a piezoelectricresonating element is supported at one end in a surface-mount package ina cantilever fashion and the package is hermetically sealed with acover, by which a piezoelectric resonator is obtained.

[0180] And, circuit parts forming an oscillation circuit areincorporated into the package forming the piezoelectric resonator, bywhich a surface-mount piezoelectric oscillator is obtained.

[0181] The manufacture of such a piezoelectric substrate as depicted inFIG. 8 involves a step of forming the masks 70 a and 70 b through whichto perform etching on both major surfaces of the flat-shapedpiezoelectric substrate 2 to form therein concavities 3 a and 3 b, and astep of performing etching on the piezoelectric substrate covered withthe masks to form the concavities 3 a and 3 b in those regions of theboth major surfaces exposed through the apertures of the masks; in themask forming step the positions of the masks 70 a and 70 b are displacedin one crystal orientation (the z-axis direction) to bring the marginaledges 3 a′ and 3 b′ of the bottoms of the concavities into alignmentwith each other.

[0182] By the way, the above piezoelectric substrate 2 may be severedfrom a piezoelectric substrate wafer having a plurality of piezoelectricsubstrates arranged in sheet form, in which case mass production bybatch process can be achieved.

[0183] [Embodiment Corresponding to Fifth Prior Art Example]

[0184] Next, a description will be given of a manufacturing methodaccording to an embodiment of the present invention which corresponds tothe fifth prior art example.

[0185]FIG. 9(a) is a perspective view of an AT-cut crystal resonatorobtained with the manufacturing method according to the presentinvention, and FIG. 9(b) is a sectional view taken on the line A-A inFIG. 9(a).

[0186] As shown, the crystal resonator 1 is a resonator that utilizesthickness-shear resonance of the AT-cut crystal resonator; the crystalresonator 1 has in its one major surface a concavity formed by chemicaletching, a resonating portion 4 formed by an ultrathin portion of theconcavity, and a thick annular portion 5 integrally formed with theouter marginal edge of the resonating portion 4 to support it.

[0187] Furthermore, on the other flat major surface of the crystalresonator 1 there are formed, by evaporation using a mask orphotolithography, a main electrode 10 a, a lead 11 a extending from themain electrode 10 a and a bonding pad 12 a, and the one major surface isvapor-deposited over the entire area thereof with an electrode 15.

[0188] It is also possible to employ a configuration in which thefull-face electrode 15 is not used but instead excitation electrodes aredisposed on both sides of the resonating portion 4 of the crystalresonator 1 in opposed relation as is the case with the FIG. 1embodiment and lead electrodes are extended from the excitationelectrodes to respective connecting pads.

[0189]FIG. 11 shows longitudinal-sectional views of the crystalresonator during etching according to the present invention, FIG. 11(a)being a longitudinal-sectional view of the crystal resonator after beingsubjected to first fine-adjustment etching, and FIG. 11(b) being alongitudinal-sectional view of the crystal resonator after beingsubjected to second main etching to second fine-adjustment etching.

[0190] The difference between the manufacturing method according to thepresent invention and the conventional method in that is not thecombined use of wet etching and dry etching, but resides in themanufacture of the crystal resonator 1 only by the wet etching processthat achieves high working efficiency and permits reduction in the costsfor equipment. The inventor of this application conducted variousexperiments and found the phenomena described below and confirmed thefeasibility of manufacturing the crystal resonator 1 of satisfactoryquality by wet etching alone.

[0191] As shown in FIG. 11(a), let a thickness error between resonatingportions 92 a and 93 a individually adjusted by the firstfine-adjustment etching so that the frequency of fundamental vibrationdetermined by the thickness of the resonating portion 4 is in the VHFband, that is, adjustment accuracy, be represented by Δ1.

[0192] On the other hand, When the crystal resonator in the state shownin FIG. 11(a) is subjected to batch adjustment by the second main tosecond fine-adjustment etching so that the frequency of fundamentalvibration dependent on the thickness of the resonating portion 4 is inthe UHF band, the thickness error between resonating portions 92 b and93 b becomes adjustment accuracy Δ2 as shown in FIG. 11(b). And theinventor of this application noticed that the adjustment accuracy Δ1 andthe adjustment accuracy Δ2 are nearly equal. That is, if the adjustmentaccuracy Δ1 is kept within a desired range by the first fine-adjustmentetching, the resonating portions need not be individually adjusted bythe second fine-adjustment etching, and all the resonating portions needonly to be collectively etched based on frequencies premeasured for someof the resonating portions 4 on the wafer.

[0193]FIG. 10 is a diagram showing a crystal resonator manufacturingprocess according to the present invention. FIG. 13 is a table showingconditions for etching in the manufacturing process.

[0194] In the first place, the major surface of a crystal wafer ispolished (step 80), and a gold/chromium film is vacuum-deposited overthe entire area of the polished major surface of the wafer (step 81). Inthis case, a crystal wafer of 80 micrometers (μm) thick is used in viewof a trade-off between the mechanical strength of the wafer and theamount of etching. Thereafter, the gold/chromium film is selectivelyremoved by lithography to form a mask patter for etching (step 82), andin the first main etching (first main etching: step 83) a crystal wafer321 having formed thereon a mask pattern 224 for etching is subjected towet etching to etch away the wafer surface portions exposed throughapertures of the mask pattern, by which resonating portions 322 a and323 a having a resonance frequency in the VHF band, for instance, 155MHz, are formed as shown in FIG. 12(a). The first main etching isconducted using a hydrogen fluoride ammonium saturated solution at 85°C., the etching rate is 25 nm/sec, and the adjustment accuracy is ±612nm/sec. In practice, since there is a variation Δ1 in the thickness ofthe resonating portions 222 a and 223 a due to a wafer working error orthe like, the resonance frequencies of the resonating portions 222 a and223 a are measured. In the first fine-adjustment etching (firstfine-adjustment etching: step 84), as shown in FIG. 21(b), theresonating portions 322 a and 323 a are individually adjusted by wetetching based on the measured frequencies so that they have desiredfrequencies. In this case, the adjustment is made to remove thethickness variation Δ1 between the resonating portions. As shown in FIG.13, the first fine-adjustment etching is carried out using a 23% or 12%dilute solution of hydrofluoric acid at room temperature (21° C. or so),the etching rate is 1.5 nm/sec or 0.75 nm/sec. Either etchant gives anadjustment accuracy of ±93 nm/sec. Which of the etchants is used for thefirst fine-adjustment etching depends on the adjustment accuracy of thefirst main etching (step 83); the thickness of each resonating portionis adjusted a value substantially within a predetermined range by usingthe 12% dilute solution of hydrofluoric acid of low etching rate whenthe adjustment accuracy is high (for example, when the variation Δ1 is±300 nm or below), and the 23% dilute solution of hydrofluoric acid ofhigh etching rate when the adjustment accuracy is not s high.

[0195] After the fist fine-adjustment etching (step 84) the resonancefrequency of each resonating portion is measured, and based on themeasured results, the etching times for the second main etching (step85) and the second fine-adjustment etching (step 86) are calculated.

[0196] For example, in the second main etching (step 85) the wafer issubjected in its entirety to wet etching to form resonating portions 322c and 323 c having a thickness of approximately 2.2 μm that correspondsto a desired resonance frequency in the UHF band, for instance, 760.9MHz as shown in FIG. 12(c); in the second fine-adjustment etching (step86) the resonating portions 322 c and 323 c are simultaneously subjectedto wet etching to form resonating portions 322 d and 323 d having thedesired frequency as shown in FIG. 12(d).

[0197] Following this, electrodes are formed by gold/nickel vacuumdeposition on either main surface of the wafer (step 87), then the waferis severed into a plurality of crystal resonators 1 (step 88), then eachcrystal resonator 1 is mounted in a package and the connecting pads 12are connected to the package by bonding or bumps (step 89), then thefinal frequency adjustment is made (step 90, and the package is sealedto form a piezoelectric resonator or piezoelectric oscillator (step 91).

[0198] The etching conditions for the second main etching and the secondfine-adjustment etching are the same as those for the first main etchingand the first fine-adjustment, respectively; particulars of theconditions are shown in FIG. 13. The adjustment accuracy in the secondmain etching and in the second fine-adjustment etching is ±110 nm/sec,but a variation in the final adjustment accuracy (±110 nm) by theetching process can be compensated for by an adjustment to the amount ofdeposition of the conductor films for the main electrode film 10 a andthe full-face electrode 15 in the gold/nickel deposition step (step 87)after etching and by high-accuracy frequency adjustment in thesubsequent final frequency adjustment step (step 90) by vapor depositionor sputtering; in actual fact, a crystal resonator having a desiredoscillation frequency is becoming a reality.

[0199] That is, in the piezoelectric resonator manufacturing methodaccording to the present invention, the first main etching and the firstfine-adjustment etching makes adjustment so that the adjustment accuracyA comes within the desired range, and the second main etching and thesecond fine-adjustment etching perform collective etching; this avoidsthe need for making a thickness adjustment for each resonating portionin the second fine-adjustment etching, hence permits simplification ofthe manufacturing process.

[0200]FIG. 14 shows the direction of etching in the manufacturing methodof the present invention; (a) is a diagram showing the shape of thecrystal resonator after being subjected to the first main etching andthe first fine-adjustment etching, and (b) is a diagram showing theshape of the crystal resonator after being subjected to the second mainetching and the second fine-adjustment etching.

[0201] As is evident from FIGS. 14(a) and (b), in the case of etchingfrom the direction indicated by the arrow, the area of the resonatingportion 90 in the horizontal direction is extremely narrow due to theanisotropy of crystal (crystal). To avoid this, after the first mainetching and the second fine-adjustment etching the major surface of thewafer subjected to etching is masked as shown in FIG. 14(c), and thewafer is subjected in its entirety to the second main etching and thesecond fine-adjustment etching from the direction indicated by the arrow(from the direction of the other major surface), by which it is possibleto obtain a resonator in which the area of the resonating portion 90inside the concavity is wider than in the past as shown in FIG. 14(d).By changing the direction of etching for the first main etching and thefirst fine-adjustment etching and for the second main etching and thesecond fine-adjustment etching as mentioned above, masking of the othermajor wafer surface (the underside in the drawing) is dispensed with.

[0202] The decision as to which of major surfaces is to be masked can bemade based on the result of the first-fine-adjustment etching (step 84)for individual adjustment.

[0203] While the present invention has been described as being appliedto the crystal resonator, the invention is also applicable to MCF(Monolithic Crystal Filter) provided with an ultrathin resonatingportion.

[0204] Besides, although the construction of the present invention hasbeen described using crystal, the invention is not limited specificallyto crystal, and it is needless to say that the invention is applicableto langasite, lithium tetraborate, lithium tantalite, lithium niobateand similar piezoelectric materials.

[0205] Furthermore, the manufacturing method of the present inventionhas been described to use the four-stage chemical etching process, butit is also possible to employ chemical etching process of four or morestages including two-stage fine-adjustment etching process with a viewto reducing the manufacture cycle time.

[0206] Moreover, The manufacturing method of the present invention hasbeen described as being applied to the crystal wafer, but the inventionis also applicable to a single blank.

[0207] Thus, it is possible to obtain a high-efficiency, low-costpiezoelectric manufacturing method, in particular, an UHF-band, AT-cutcrystal resonator manufacturing method.

[0208] As described above, according to the inventions corresponding toclaims 1 to 5, in the case where an ultraminiature piezoelectricsubstrate, which has a resonating portion made by forming a concavity byetching in the surface of a piezoelectric substrate made of ananisotropic piezoelectric crystal material, is mass-produced by batchoperating using a large-area piezoelectric substrate wafer, the annularmarginal portion surrounding the concavity can be formed thick enough toprevent cracking when the wafer is severed. Further, a conductive traceis routed along a gentle slope formed on the inner wall of the annularportion, by which a break in the trace can be avoided. Besides, in thecase of supporting the piezoelectric resonating element in a cantileverfashion in a package, the application of stress by the weight of thecrystal resonating element to the resonating portion can be prevented byspacing the cantilever supporting portion and the resonating portion asfar apart as possible. Thus, the first object of the invention is toimplement the optimum configuration of an ultraminiaturizedpiezoelectric substrate having an ultrathin resonating portion and athick annular marginal portion surrounding it.

[0209] According to the second inventions corresponding to claims 6 to12, in the case where a through hole (concave notches) for electricalconnection between two connecting pads on both sides of each substrateis formed by chemical etching in a piezoelectric substrate wafer,insufficient formation of the through hole by limitations on upsizing ofits opening and the resulting decrease in productivity can be prevented.

[0210] According to the third inventions corresponding to claims 13 to19, in order to obviate various glitches caused when the thicknessadjustment of the resonating portion for each concavity is made byetching for a different period of time after the formation of theconcavities in a piezoelectric substrate wafer by one operation,fine-adjustment of the thickness of each resonating portion can be madeby etching on the flat surface side of the wafer instead of makingthickness adjustment by filling with each concavity with an etchant.

[0211] According to the inventions corresponding to claims 20 to 26, ina piezoelectric substrate of an anisotropic piezoelectric material whichhas a thin resonating portion is formed by concavities made by chemicaletching in both major surfaces of the substrate, it is possible toprevent the effective vibrating area from becoming small due to the factthat the positions of the both concavities disposed opposite across theresonating portion are displaced in the one crystal orientation relativeto each other.

[0212] According to the invention recited to claim 27, it is possible toobtain a piezoelectric element manufacturing method which is simple inmanufacturing process and low-cost in equipment, and the piezoelectricelement produced by this method is a high-quality element that has nochemical reaction layer on its surface but is not etched more thanrequired, and the manufacturing method allows ease in control forconstant etching rate and achieves high production efficiency bycontinuous etching.

[0213] According to the inventions recited in claims 28 to 34, it ispossible to obtain a manufacturing method that suppresses reduction inthe area of the resonating portion which is caused by the dependence ofetching rate on the crystal orientation.

What is claimed is:
 1. A piezoelectric substrate made of an anisotropicpiezoelectric crystal material and comprising a thin resonating portion,and a thick annular portion integrally surrounding the outer marginaledge of said resonating portion to form a concavity in at least one ofmajor surfaces of said piezoelectric substrate, characterized in that:the inner wall of said annular portion gently slopes in the one crystalorientation more than in the other crystal orientation perpendicularthereto; and said piezoelectric substrate is longer in said one crystalorientation than in said other orientation.
 2. A piezoelectric substratemade of an AT-cut crystal material and comprising a thin resonatingportion, and a thick annular portion integrally surrounding the outermarginal edge of said resonating portion to form a concavity in at leastone of major substrate surfaces of said piezoelectric substrate,characterized in that: said piezoelectric substrate made of said AT-cutcrystal material is longer in a z′-axis direction than in an x-axisdirection.
 3. A piezoelectric resonating element comprising excitationelectrodes formed on both sides of said resonating portion in thepiezoelectric substrate of claim 1 or 2 in opposed relation, leadelectrodes extending from the excitation electrodes to one marginal edgeof the piezoelectric substrate lengthwise thereof, and connecting padsconnected to the lead electrodes, respectively, characterized in thatthe lead electrode extending from the excitation electrode formed on theside of said concavity is routed along said gently sloping inner wall ofthe annular portion.
 4. A piezoelectric resonator, characterized in thatthe piezoelectric substrate forming the piezoelectric resonating elementof claim 3 is supported at one end lengthwise thereof in a cantileverfashion in a surface-mount package.
 5. A surface-mount piezoelectricoscillator, characterized by the provision of at least the piezoelectricresonator of claim 4, and an oscillation circuit.
 6. A piezoelectricsubstrate comprising a thin resonating portion, and a thick annularportion integrally surrounding the outer marginal edge of saidresonating portion to form a concavity in at least one of major surfacesof said piezoelectric substrate, one side of said annular portion beingextended to form a jut-out portion, characterized in that: at least oneconcave notch open to both surfaces of the piezoelectric substrate isformed in a forward marginal edge of said jut-out portion.
 7. Thepiezoelectric substrate of claim 6, characterized in that said concavenotch is formed in one of both end corner portions of the forwardmarginal edge of said jut-out portion.
 8. A piezoelectric resonatingelement comprising excitation electrodes formed on both sides of saidresonating portion in the piezoelectric substrate of claim 6 or 7 inopposed relation, lead electrodes extending from the excitationelectrodes to the forward marginal edge of said jut-out potion,characterized in that either one of the lead electrodes is routed viasaid concave notch to the opposite substrate surface and connected to aconnecting pad formed on said opposite substrate surface.
 9. Apiezoelectric resonator, characterized in that two connecting padsdisposed side by said on the same surface of the jut-out portion of thepiezoelectric substrate forming the piezoelectric resonating element ofclaim 8 are respectively bonded by a conductive adhesive to pads in asurface-mount package.
 10. A surface-mount piezoelectric oscillator,characterized by the provision of at least the piezoelectric resonatorof claim 9 and an oscillation circuit.
 11. A construction of apiezoelectric substrate wafer having the piezoelectric substrates ofclaims 6 to 8 arranged in sheet form, characterized in that said concavenotch is formed by making a through hole in the wafer astride adjacentpiezoelectric substrates to simultaneously form such concave notches inthe both piezoelectric substrate.
 12. A piezoelectric substrate waferhaving a plurality of piezoelectric substrates of claims 6 or 7 arrangedin sheet form, characterized in that: an unused region is disposedbetween said adjacent piezoelectric substrates; and said concave notchis formed in one piezoelectric substrate by making a through holeastride the piezoelectric substrate and the unused region adjacentthereto.
 13. A piezoelectric substrate comprising a thin resonatingportion, and a thick annular portion integrally surrounding the outermarginal edge of said resonating portion to form a concavity in at leastone of major surfaces of said piezoelectric substrate, characterized inthat: a resonating portion thickness fine-adjustment portion is formedon the substrate surface opposite to said concavity.
 14. A piezoelectricresonating element, characterized by the provision of excitationelectrodes formed on both sides of said resonating portion of thepiezoelectric substrate of claim 13 in opposed relation, lead electrodesextending from the excitation electrodes to the forward marginal edge ofthe piezoelectric substrate lengthwise thereof, and connecting padsrespectively connected to the lead electrodes.
 15. A piezoelectricresonator, characterized in that the piezoelectric substrate forming thepiezoelectric resonating element of claim 14 is supported at one end ina cantilever fashion in a surface-mount package.
 16. A surface-mountpiezoelectric oscillator, characterized by at least the piezoelectricresonator of claim 15 and an oscillation circuit.
 17. A piezoelectricsubstrate wafer, characterized in that a plurality of piezoelectricsubstrates of claim 13 are arrange in sheet form.
 18. The piezoelectricsubstrate wafer of claim 17, characterized in that a dead space isinterposed between adjacent piezoelectric substrates by two paralleldividing grooves.
 19. A piezoelectric substrate wafer manufacturingmethod, characterized in that the resonating portion thicknessfine-adjustment portion formed on the substrate surface on the oppositeside from the concavity of each piezoelectric substrate of thepiezoelectric substrate wafer of claim 17 or 18 is formed by filling anetchant into each aperture of a guide mask held against said oppositesurface side of the piezoelectric substrate wafer on said opposite sidesurface thereof, said guide mask having arranged therein in a gridpattern a plurality of apertures each larger than the concavity.
 20. Apiezoelectric substrate made of an anisotropic piezoelectric crystalmaterial and comprising a thin resonating portion, and a thick annularportion integrally surrounding the outer marginal edge of saidresonating portion to form concavities in both major surfaces of saidpiezoelectric substrate, characterized in that: the inner wall of saidannular portion defining each of said concavities gently slopes in theone crystal orientation more than in the other crystal orientationperpendicular thereto; and the positions of those of marginal edges ofthe bottoms of said concavities lying in said one crystal orientationare aligned with each other.
 21. The piezoelectric substrate of claim20, characterized in that it is made of an AT-cut crystal material. 22.A piezoelectric resonating element, characterized by the provision ofexcitation electrodes formed on both sides of said resonating portion inthe piezoelectric substrate of claim 20 or 21 in opposed relation, leadelectrodes extending from the excitation electrodes to one marginal edgeof the piezoelectric substrate lengthwise thereof, and connecting padsconnected to the lead electrodes, respectively.
 23. A piezoelectricresonator, characterized in that the piezoelectric substrate forming thepiezoelectric resonating element of claim 22 is supported at one endlengthwise thereof in a cantilever fashion in a surface-mount package.24. A surface-mount piezoelectric oscillator, characterized by theprovision of the piezoelectric resonator of claim 23, and an oscillationcircuit.
 25. A method for the manufacture of a piezoelectric substratewhich is made of an anisotropic piezoelectric crystal material andcomprises a thin resonating portion, and a thick annular portionintegrally surrounding the outer marginal edge of said resonatingportion to form concavities in both major surfaces of said piezoelectricsubstrate, and in which the inner wall of said annular portion definingeach of said concavities gently slopes in the one crystal orientationmore than in the other crystal orientation perpendicular thereto, saidmethod comprising: a mask forming step of covering the both majorsurfaces of a flat-shaped piezoelectric substrate with masks for formingtherein said concavities; and a concavity forming step of performingetching on the piezoelectric substrate covered with said masks to formconcavities in the both major surfaces of the piezoelectric substrateexposed through apertures of the masks; characterized in that: thepositions of the masks are displaced relative to each other in said onecrystal orientation to bring marginal edges of the bottoms of saidconcavities into alignment.
 26. The piezoelectric substratemanufacturing method of claim 25, characterized in that saidpiezoelectric substrate is a piezoelectric substrate wafer having aplurality of piezoelectric substrates arranged in sheet form.
 27. Amethod for the manufacture of a piezoelectric resonating element havinga thin resonating portion formed by making a concavity in one of majorsurfaces of a piezoelectric substrate, said method comprising: a firstmain etching step of etching away predetermined portions of one of majorsurfaces of a piezoelectric wafer to form resonating portions; afrequency measuring step of measuring resonance frequencies of saidresonating portions; a first fine-adjustment etching step of making fineadjustments to the thicknesses of said resonating portions based on thefrequencies measured by said frequency measuring step; a second mainetching step of further reducing the thicknesses of said resonatingportions; and a second fine-adjustment etching step of making fineadjustments to the thicknesses of said resonating portions;characterized in that either of said etching steps is a wet etchingstep.
 28. The piezoelectric resonating element of claim 27,characterized in that said second main etching steps is a step ofperforming etching on said piezoelectric wafer over the entire area ofthe other major surface thereof.
 29. The piezoelectric resonatingelement manufacturing method of claim 27, characterized in that saidsecond main etching step is a step of performing etching on saidpiezoelectric wafer over the entire area of either major surfacethereof.
 30. The piezoelectric resonating element manufacturing methodof any one of claims 27 to 29, characterized in that said secondfine-adjustment etching step is a step of performing etching on saidpiezoelectric wafer over the entire area of the other major surfacethereof.
 31. The piezoelectric resonating element manufacturing methodof any one of claims 27 to 29, characterized in that said second mainetching step is a step of performing etching on said piezoelectric waferover the entire area of either major surface thereof.
 32. Thepiezoelectric resonating element manufacturing method of any one ofclaims 27 to 31, characterized in that it further comprises a step ofobtaining a plurality of piezoelectric resonating elements from onepiezoelectric wafer by severing the wafer into individual piezoelectricresonating elements after forming a plurality of concavities in thewafer.
 33. The piezoelectric resonating element manufacturing method ofclaim 31, characterized in that said frequency measuring step of claim27 is a step of making a frequency measurement for all of the resonatingportions, and that said firs fine-adjustment etching step is a step ofperforming etching for each of said resonating portions.
 34. Thepiezoelectric resonating element manufacturing method of claim 32 or 33,characterized in that the frequency measuring step in claim 27 is a stepof measuring frequencies of some of the plurality of resonatingportions, and that said second main etching step and said secondfine-adjustment etching step are steps of simultaneously performingetching all the resonating portions.